1. Technical Field of the Invention
The present invention relates to an explosion-proof safety valve assemblage having a lead cap capable of rapidly breaking the current of a closed battery and releasing gas when the internal pressure of the battery is elevated due to the gas generated by overcharging the battery or due to a short circuit in the battery, and relates to a closed secondary battery using it.
2. Prior Art
Recently with increasing demand for electronic devices that are made much smaller in size, are portable and have enhanced performance, non-aqueous electrolyte secondary batteries with high energy density have been popularly used as power sources for them. Since the non-aqueous electrolyte secondary battery uses an alkali metal like lithium as an active material for positive and negative electrodes thereof, it employs a container of sealed structure so that such alkali metal may not react with water in the atmosphere.
Such a non-aqueous electrolyte secondary battery of sealed structure has a merit of storability, but reversely, there arises a problem due to the very high sealability. Namely, if exposed to high temperature or mishandled, such closed secondary battery using non-aqueous electrolyte will suffer an abnormal elevation of its own internal pressure and even a burst of itself finally. For example, if a lithium secondary battery is supplied with more than a rated current and overcharged, or the battery is mishandled thereby to cause a short circuit of the positive electrode and the negative electrode, inducing a large current into the battery, the non-aqueous electrolyte in the electrode element is sometimes decomposed to generate gas. If such gas fills the container of sealed structure mentioned above, the internal pressure of the battery is abnormally elevated and the battery at last bursts itself.
Accordingly, an electronic device using such non-aqueous electrolyte secondary battery as an electric source may suffer damage.
So far, there is known a closed secondary battery using non-aqueous electrolyte which is provided with an explosion-proof mechanism for preventing such burst of battery as mentioned above.
For example, Japanese Patent Laid-open Publication No. HEI-6-215760 discloses an explosion-proof mechanism of a closed non-aqueous electrolyte secondary battery, which is provided with a valve diaphragm disposed above an electrode element accommodated in a cylindrical outer container of a battery having a bottom and provided with a lead for breaking current disposed above the valve diaphragm. When the internal pressure of the battery is elevated, the above mentioned valve diaphragm is transformed upward so as to rupture the lead for breaking current to cut the current and at substantially the same time the valve diaphragm ruptures to release the internal pressure of the battery.
However, such closed non-aqueous electrolyte secondary battery provided with the conventional explosion-proof mechanism has a problem. Namely, it is necessary for such battery to have a lead plate or lead wire precisely worked in order to attain reliable breaking of the current thereof. It is also necessary to pay attention-in particular to the positioning of the lead plate or lead wire when it is being assembled into a battery in the manufacturing operations. Thus, the manufacturing process for this type of secondary battery should include a non-negligible amount of man-hour in the process in order to obtain a desired mechanism.
Therefore, it is an object of the present invention to provide an explosion-proof safety valve assemblage and a closed secondary battery using it, in which a lead cap is integrally formed with an explosion-proof safety valve in place of the conventional lead plate or lead wire for composing a current circuit of the battery, so that some of the manufacturing man-hour in the process such as precision working and positioning of the lead plate or lead wire, which are required in the manufacturing process of the conventional closed secondary battery, can be deleted. According to the present invention, it is also possible to improve the yield of the product and reduce the manufacturing cost thereof
To achieve the above mentioned object in a first embodiment, there is provided an explosion-proof safety valve assemblage for use in a closed secondary battery comprising an outer container, an electrode element, consisting of a positive electrode, negative electrode, and a separator, accommodated in said outer container, wherein said explosion-proof safety valve assemblage is sealingly secured to an open end portion of said outer container via an insulating gasket integrally with a closing cap, and said safety valve assemblage consists of a safety valve element, a coreless disc-shaped insulator, and a lead cap formed of a metal substrate substantially the center of which is provided with an opening laminated with a metal foil.
In a second embodiment there is provided another explosion-proof safety valve assemblage for use in a closed secondary battery comprising an outer container, an electrode element, consisting of a positive electrode, negative electrode and a separator, accommodated in said outer container, wherein said explosion-proof safety valve assemblage is sealingly secured to an open end portion of said outer container via an insulating gasket integrally with a closing cap, and said safety valve assemblage consists of a safety valve element, substantially the center of which is provided with a concavity, a coreless disc-shaped insulator, and a lead cap formed of a metal substrate laminated with a metal foil, substantially the center of which is provided with a projection.
According to claim 3, in the explosion-proof safety valve assemblage as claimed in the preceding claims, said concavity of the safety valve element and said projection of the lead cap, are laser welded or ultrasonically welded so that a current circuit is produced in the closed second battery.
According to claim 4, in the preceding safety valve assemblage, said lead cap is provided with a plurality of through holes for gas releasing.
According to claim 5, in the preceding safety valve assemblage, said safety valve element is formed of a metal substrate laminated with a metal foil.
According to claim 6, there is provided a closed secondary battery employing the above mentioned explosion-proof safety valve assemblage.